This invention relates generally to fire detection and extinguishing equipment, and more particularly, to methods and apparatus for confirming the presence of a fire or of smoldering materials.
A modem addressable fire alarm control panel (FACP) may have one or more channels of addressable initiating devices for automatic detection of fire. An automatic alarm may be initiated by smoke sensors, heat sensors, or other sensors. These sensors may be analog or digital sensors, but most frequently are analog sensors. The sensors constantly measure ambient conditions and report changes back to the FACP that relate to smoke obscuration, carbon monoxide (CO) content, temperature, etc., depending on the sensor type. For example, a smoke sensor might send back a “clean air” analog value of “72,” which, for this particular sensor, may represent a smoke obscuration of 0% per foot. An alarm value for smoke sensors is often set to an obscuration percentage of 2.5% or 3.5% per foot. An alarm analog value is usually represented by a higher analog value than the clean air value. A scale can be used to relate the analog readings to percent smoke obscuration. For example, one known automatic alarm system uses 27 least significant bit (LSB) “ticks” to represent 1% smoke obscuration (i.e., a value of “99”).
Because the alarm value is typically set to 2.5% or 3.5%, the sensor reading required for an alarm may be in the range of 139-166. The setting of a higher alarm threshold tends to prevent dispatching the fire department to investigate nuisance conditions that are not real alarms. Because of the very strong desire to avoid false alarms, very early detection of smoldering material, for example, may sometimes be limited. Some methods have been introduced to reduce the occurrence of false alarms, however, such methods require additional smoke or smoke for a longer period than is required for early detection of smoldering material for an alarm to be initiated. Other methods in which multiple fixed sensors are used to make early alarm decisions are limited in that only one sensor may be close enough to the source to detect early stage combustion.